1. Field of the Invention
The invention relates to a valve system of a V-type engine in which a camshaft provided in each bank of the engine is formed with valve cams for driving (i.e., opening and closing) engine valves and a pump cam for driving a fuel pump that feeds fuel under pressure to a fuel injection device.
2. Description of Related Art
There is known a valve system in which a fuel pump that delivers fuel under pressure to a fuel injection device is driven by a camshaft on which valve cams for driving (i.e., opening and closing) engine valves, such as the intake valves or the exhaust valves, are formed. In the known valve system, a piston of the fuel pump is urged by a spring, or the like, into contact with a pump cam formed on the camshaft, so that rotating the pump cam causes the piston to reciprocate in the fuel pump, namely, the pump cam drives the piston as it rotates with the camshaft. With the piston thus reciprocating, the fuel is drawn from the fuel tank into the fuel pump, and is then pressurized and fed to the fuel injection device. Meanwhile, the camshaft on which the pump cam as well as the valve cams are formed is subjected to torque fluctuations in driving the fuel pump, in addition to torque fluctuations in driving the engine valves. If the driving torque fluctuations associated with the fuel pump are superimposed on those associated with the engine valves to increase the amplitude of total torque fluctuations, excessive tension is applied to a drive member, such as a timing belt or a timing chain, for driving the camshaft, which may result in a reduction of the service life of the drive member. In view of this situation, a valve system has been proposed in which the phase relationship (i.e., relationship in the angular position) between the valve cams and the pump cam is set to suppress or reduce fluctuations in driving torque experienced by the camshaft, as disclosed in, for example, JP-A-H10-176508. The publication also discloses an example of a V-type six-cylinder engine in which the valve system as described just above is applied to a camshaft provided in each bank.
In the V-type six-cylinder engine as disclosed in the above-identified publication, crank-angle phase differences among the cylinders provided in each bank are set at equal intervals, namely, the pistons of the cylinders in each bank move with equal phase shifts in terms of the crank angle during operation of the engine. Therefore, torque fluctuations in driving the engine valves occur in substantially the same form at regular intervals while the camshaft makes one revolution or while the crankshaft makes two revolutions. It is, therefore, relatively easy to set the phase (angular position) of the pump cam with respect to the valve cams to suppress the driving torque fluctuations arising in the camshaft. In some types of V-type engines, such as a V-type eight-cylinder engine, however, crank-angle phase differences among the cylinders in each bank may not be set at equal intervals. In such cases, the camshaft of each bank is subjected to a complicated form of torque fluctuations in driving the engine valves, which makes it difficult to appropriately set the timing and frequency of driving the fuel pump.